Among the most frequent diseases with fatal consequences are diseases of the vascular vessels, in particular heart infarcts. This is caused by diseases of the coronary vessels, so-called arteriosclerosis. With this, the diameter of the vessels is reduced by deposits (arteriosclerotic plaque) on the vessel walls to the point where an individual coronary blood vessel, or several, may be blocked. It has now been recognized that the danger of suffering a coronary infarct does not depend primarily on the reduction in the diameter of the vessels. Rather, it depends on whether the thin protective layer, which covers the arteriosclerotic deposits, remains intact. If this ruptures, blood platelets preferentially deposit at the breakage site, and within a short time these completely close up the blood vessel and thus cause a coronary infarct.
A narrowing, also referred to as stenosis, of the coronary vessels, as a possible precursor of a blockage which may lead to a coronary infarct, may be diagnosed for example in the course of coronary angiography. During the latter it is normal to introduce into the coronary vessels a catheter, through which an X-ray contrast agent is injected into the region of the vessels to be investigated. An X-ray photograph of the region of the vessels is then prepared, and the catheter is removed again. The inner space within the blood vessels, which is filled with the contrast agent, then shows up on the X-ray image. The resulting image is also referred to as an angiogram. One disadvantage of this method consists in the fact that it only shows the diameter of the vessels which is usable by the blood flow, or the point of narrowing, as a silhouette. It is not possible to make any statement about the deposits, in particular their thickness or the degree of the inflammatory process.
Recently is has also become possible, for the purpose of more precisely diagnosing a stenosis, to introduce into the coronary vessels a so-called ultrasonic catheter with an imaging intravascular ultrasonic sensor (IVUS sensor). An ultrasonic catheter of this type is, for example, known from DE 198 27 460 A1. The IVUS sensor supplies ultrasonic images from the interior of the vessel, whereby the image formed is normally a 360° sectional view of the vessel wall and also the underlying tissue layers.
Alternatively, the investigatory catheter can also be equipped with a sensor for optical coherence tomography (OCT) or with a sensor for intravascular magnetic resonance tomography (IVMRI=intravascular magnetic resonance imaging).
Optical coherence tomography imaging supplies high-resolution images which, in particular, show the structures in the region of the surface of the vessel comparatively exactly. The principle underlying this method is that the catheter radiates light fed via an optical guide, preferably infrared light, into the vessel, with the light reflected by the latter being coupled back into the optical waveguide and fed to an analysis device. In the analysis unit, the coherence of the reflected light is analyzed against the reference light—in a way similar to that in a Michelson interferometer—to generate an image. An OCT investigatory catheter is known, for example, from U.S. Pat. No. 5,921,926. With the OCT method, the section of the vessel to be investigated must be briefly cleared of blood. For this purpose, the blood flow is normally interrupted during the image capture by a closure plug, and the section of the vessel washed out using a physiological saline solution.
Another imaging method, which is known in particular for its good representation of soft tissues, is magnetic (core) resonance tomography. With this method, the magnetic moments (core spins) of the nuclei of atoms in the tissue to be investigated are aligned in an external magnetic field and excited by irradiated radio waves into a gyroscopic movement (precession), whereby relaxation processes induce in an associated receiving coil an electrical magneto-resonance signal which forms the basis for calculating an image. There has recently been success in miniaturizing the elements which generate the magnetic field, and integrating them into an imaging IVMRI sensor in an investigatory catheter, in a way which enables the intracorporal or intravascular use, as applicable, of the MRI method, whereby the necessary static magnetic field is also generated or applied, as appropriate, within the patient's body. A solution of this type is described, for example, in U.S. Pat. No. 6,704,594 B1.
If, using the investigatory methods outlined above, stenoses of the coronary vessels are recognized which are a threat to the patient or greatly limit their capabilities, further treatment steps are generally necessary. Depending on the case, this will involve carrying out either a bypass operation or a balloon dilatation, also known as a “percutaneous transluminal coronary angioplasty (PTCA)”. Nowadays, it is preferable to use the PTCA method. With this, the narrowings of the coronary vessels are dilated using a so-called balloon catheter, which is introduced under X-ray control into the region to be treated. In the region of its front tip (at the distal end) this catheter has a balloon which can be expanded, generally using a saline solution under pressure, which is expanded or inflated at the site of the stenosis. So that the enlargement of the vessel does not revert to its original state (restenose), a so-called stent is frequently introduced into the widened section of the blood vessel after the dilation. This stent is a cylindrical mesh, generally metallic, which is plastically reshaped using the balloon, and lies against the inner wall of the vessel when it is expanded.
Even after the implantation of a stent, restenoses can occur. Reasons for postinterventional restenosis are the continuation of the causative arteriosclerosis, and the response of the vessel wall to the PTCA-induced trauma. For this reason, techniques are now available to treat the section concerned of the vessel prior to the implantation of a stent, and thus prevent restenosis. For example, clinical studies have shown that endovascular irradiation of the vessel wall with beta and/or gamma radiation (brachytherapy) reduces restenoses. The mechanisms are not yet fully clarified, but various models are being discussed, e.g. cell death, cell inactivation, inhibition of cell migration, suppression of the vascular structural remodeling, and blocking of the extracellular matrix synthesis. The disadvantage of brachytherapy lies in the additional radiation load on the patient and in the expensive logistic process which the radioactive sources necessitate in the clinic (procurement, storage, disposal).
A new method is currently in clinical research: pretreatment of the stenosis before or during dilatation, with the help of cryotechnology (cryogenics). In this case, a so-called cryocatheter is fed into the vessel as far as the stenosis. As soon as the stenosis is reached, liquid nitrogen is introduced into the catheter, this reaching the gaseous state by the time it arrives at the catheter tip, and when it expands it blows up a dilation balloon. The nitrogen thus functions as both an expansion medium and a coolant. In other words, the section of the blood vessel which is affected by the stenosis if briefly cooled to a very low temperature and, at the same time, is widened by the stretching of the balloon. The cold-induced “sclerosis” of the tissue achieves a similar effect in the vessel wall as with brachytherapy, i.e. the restenosis rate is significantly reduced, but the patient is exposed to no additional radiation load. This method is described, for example, by James D. Joye et al. in “In Vivo Study of Endovascular Cryotherapy for the Prevention of Restenosis”, which can be ordered on the Internet under http://www.cryoinc.com. A cryocatheter which is suitable for carrying out the method is known, for example, from U.S. Pat. No. 6,355,029 B1.
All in all, it is a disadvantage that the known procedure is multiply invasive, i.e. several different catheters must be introduced—for preliminary investigation and for treatment—one after another. However, each invasion has a certain associated risk for the patient. The fact that the investigatory catheter must first be withdrawn again from the patient's vascular system, before the cryocatheter provided for the cryotherapy and widening of the blood vessel can be introduced, means that problem are frequently presented in locating again exactly the treatment region determined during the preliminary investigation. Specifically, when it is being introduced the cryocatheter can only be observed in outline in the angiography X-ray image, making its manipulation more difficult and the position of the catheter tip within the section of the vessel which is to be treated can only be approximately estimated. It is possible that the cryocatheter is not precisely positioned in the target area during its use, thereby increasing the risk of restenosis.